Multi-core optical fiber

Abstract

A multicore optical fiber of one embodiment has a structure for simultaneously achieving low transmission loss, low splicing loss, and low inter-core crosstalk. The multicore optical fiber includes a plurality of cores each of which is made of silica glass; a common cladding which is made of silica glass and surrounds the plurality of cores; and a resin covering which surrounds the common cladding. The common cladding has a refractive index lower than that of the plurality of cores. The plurality of cores each contain at least 10,000 ppm of chlorine in at least a part thereof.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a multi-core optical fiber (hereinafter, referred to as "MCF").

[0002] This application claims priority based on Japanese Patent Application No. 2020-177977 filed on October 23, 2020, the contents of which are incorporated herein by reference in its entirety and is incorporated herein by reference in its entirety. BACKGROUND

[0003] A space multiplexing optical fiber in which a plurality of space channels are provided inside is attracting attention as a solution to expand the transmission capacity of a transmission path in which there is a spatial restriction. Among them, for an MCF having a structure in which a plurality of cores having a higher refractive index than a cladding having a prescribed refractive index are included in the cladding, since each core can be designed to maintain interchangeability with existing optical fibers, it has the advantage of being low in introduction cost.

[0004] An MCF is a particularly effective transmission medium in a transmission path in which there is a spatial restriction but the demand for transmission capacity is large. One of representative examples of such a transmission path is a submarine cable. If the outer diameter of a submarine cable is made thick, the manufacturing cost and the laying cost of the cable itself increase. Therefore, it is disclosed in Non-Patent Literature 1 that the outer diameter of the cable is limited, and the number of optical fibers that can be accommodated is typically around 32. On the other hand, as the expansion of data utilization, the demand for transmission capacity of a submarine cable increases exponentially, and the necessity of a cable that can accommodate more than the above-mentioned 32 is pointed out in Non-Patent Literature 2.

[0005] As an MCF suitable for a submarine cable, for example, there is a dual-core optical fiber disclosed in Non-Patent Literature 3. The dual-core optical fiber has two cores in a cladding having an outer diameter of 125 μm. The two cores are each composed of a center core to which a little fluorine is added, a ring core composed of pure silica, and an inner cladding to which fluorine is added. In addition, the two cores are arranged with a center-to-center distance of 50 μm in a state of being surrounded by a common cladding in which fluorine is also added. According to the above-mentioned Non-Patent Literature 3, it is described that the inter-core crosstalk when signals are propagated toward the two cores can be -40 dB / 100 km or less. This corresponds to -17 dB / 100 km or less of inter-core crosstalk when propagating in the same direction assuming a repeater span length of 50 km. The dual-core optical fiber has a cable cutoff wavelength of 1530 nm or less by the two cores, thereby operating in a single mode at a wavelength of 1530 nm or more, and the transmission loss is suppressed to be low at a wavelength of 1550 nm, 0.162 dB / km.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Document 1: U.S. Patent No. 9,618,692

[0009] Non-patent literature

[0010] Non-patent document 1: Florence Palacios, et al., "Ultra-Large Effective AreaFibre Performances in High Fiber Count Cables and Joints, A New Technical Challenge," Proceedings of SubOptic 2016, TU1A-2 (2016).

[0011] Non-patent document 2: Herve Fevrier, et al., "Facebook Perspective on Submarine Wet Plant Evolution," Proceedings of OFC 2019, M2E.4 (2019).

[0012] Non-patent document 3: Yoshiaki Tamura, et al., "Low-Loss Uncoupled Two-Core Fiber for Power Efficient Practical Submarine Transmission," Proceedings of OFC2019, M1E.5 (2019). Summary of the Invention

[0013] The MCF (multi-core fiber) disclosed herein comprises multiple cores, a common cladding, and a resin coating. The multiple cores are each made of silica glass. The common cladding is made of silica glass and surrounds the multiple cores. Furthermore, the common cladding has a refractive index lower than that of each of the multiple cores. The resin coating surrounds the common cladding. In particular, each of the multiple cores contains 10,000 ppm or more of chlorine in at least a portion thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of an MCF according to an embodiment of the present disclosure.

[0015] Figure 2 It is a schematic diagram showing the cross-sectional structure, Cl concentration, F concentration, and refractive index curve of the glass fiber included in the MCF according to the first embodiment of the present disclosure.

[0016] Figure 3FIG. 1 is a schematic view of a cross-sectional structure, a Cl concentration, an F concentration, and a refractive index profile of a glass fiber included in an MCF according to a second embodiment of the present disclosure. DETAILED DESCRIPTION

[0017] [Technical Problem to be Solved by the Disclosure]

[0018] The inventors have studied the above-described prior art, and as a result, have found the following technical problem. That is, in the above-described Non-Patent Literature 2, the outer diameter of the cladding resin of the optical fiber is reduced from the existing 250 μm to 200 μm is disclosed as one of the solutions, but the improvement of the space density by this is at most 1.4 times, and moreover, the mechanical reliability is reduced because the cladding thickness is reduced. Therefore, according to the solution disclosed in Non-Patent Literature 2, there is a possibility that an additional cost for ensuring sufficient reliability is generated.

[0019] In addition, in the dual-core optical fiber of the above-described Patent Literature 3, there is a technical problem that the connection loss at the time of fusion splicing easily becomes high. This is due to the fact that if the center-to-center distance of the two cores is 50 μm, the outer diameter of the cladding is larger than 125 μm. The viscosity of the end of the optical fiber that is cut at the time of fusion splicing is reduced by heating, but at this time, the end of the outer peripheral portion of the cladding is deformed in a manner that the edge is rounded due to the surface tension. Due to this, in the outer peripheral portion of the cladding, the waveguide structure inside also becomes easily deformed. In addition, at the time of cleaving and cutting the optical fiber, in the outer peripheral portion of the cladding, the stress is unstable (a state in which the stress applied along the outer periphery of the cladding is not constant), and there is a possibility that a crack will travel non-perpendicularly with respect to the central axis of the optical fiber (optical fiber axis). Such non-planarization of the end face also becomes a cause of the fusion splicing loss. Furthermore, in the case of fusion splicing two optical fibers, by rotating the two optical fibers that should be connected with the optical fiber axis as the center, respectively, the rotation angles are aligned in a manner that the core arrangement of the two optical fibers becomes the same orientation. At this time, there is also a technical problem that the larger the center-to-center distance of the two cores, the larger the positional displacement of the cores caused by the rotation angle error.

[0020] On the other hand, if the center-to-center distance of the two cores is reduced in order to reduce the fusion splicing loss, the crosstalk between the cores increases as disclosed also in the above-described Non-Patent Literature 3. By increasing the refractive index difference between the core and the cladding, it is also possible to reduce the crosstalk between the cores, but in the prior art, in order to achieve low transmission loss, it is necessary to use pure silica glass or silica glass to which fluorine is added at a low concentration of 3000 ppm or less, but in this case, the refractive index of the core becomes low. In order to increase the refractive index difference between the core and the cladding, it is necessary to reduce the refractive index of the cladding, but if the fluorine concentration of the cladding is increased, there is a technical problem that an increase in the transmission loss and an increase in the manufacturing cost are generated thereby.

[0021] The present disclosure has been made to solve the above-described technical problem, and has an object to provide a MCF having a structure for simultaneously achieving low transmission loss, low fusion splice loss, and low inter-core crosstalk.

[0022] [Effects of the present disclosure]

[0023] According to the multicore optical fiber of the present disclosure, low transmission loss, low fusion splice loss, and low inter-core crosstalk can be simultaneously achieved.

[0024] [Explanation of embodiments of the present disclosure]

[0025] First, the contents of the embodiments of the present disclosure are individually and separately explained.

[0026] (1) The MCF (multicore optical fiber) of the present disclosure, as one way thereof, has a plurality of cores, a common cladding, and a resin coating. The plurality of cores is each composed of silica glass. The common cladding is composed of silica glass and surrounds the plurality of cores. In addition, the common cladding has a refractive index lower than the refractive index of each of the plurality of cores. The resin coating surrounds the common cladding. In particular, the plurality of cores each contains 10,000 ppm or more of chlorine in at least a part thereof. According to this structure, low transmission loss and low inter-core crosstalk can be simultaneously achieved.

[0027] (2) As one way of the present disclosure, in each of the plurality of cores and the core-cladding boundary of the common cladding, the concentration of the chlorine is preferably 2,000 ppm or less. In addition, in a migration region of each of the plurality of cores including the core-cladding boundary, the concentration of the chlorine is preferably continuously reduced toward a direction away from the refractive index center of each of the plurality of cores. In this case, the generation of bubbles can be suppressed, and the productivity of the parent material and the optical fiber can be improved.

[0028] (3) As one way of the present disclosure, the common cladding preferably contains fluorine. In addition, in a connection region of the common cladding, the concentration of the fluorine is preferably 7,500 ppm or less. In addition, the concentration of the fluorine is preferably continuously reduced toward a direction close to the refractive index center of each of the plurality of cores. In this case, low inter-core crosstalk can be achieved.

[0029] (4) As one way of the present disclosure, in the core-cladding boundary, the concentration of the fluorine is preferably 11,000 ppm or less. In this case, the generation of bubbles can be suppressed, and the productivity of the parent material and the optical fiber can be improved.

[0030] (5) As one way of the present disclosure, the common cladding can have a first common cladding directly surrounding each of the plurality of cores, and a second common cladding surrounding the first common cladding. In addition, the average concentration of the fluorine in the first common cladding is preferably 900 ppm or more greater than the average concentration of the fluorine in the second common cladding. In this case, the residual of the drawing tension on each core can be suppressed, and low transmission loss and a high relative refractive index difference can be achieved.

[0031] (6) As one embodiment of the present disclosure, the MCF can also include two cores as the plurality of cores. In addition, it is preferable that the inter-core distance in the two cores be 30 μm or more and 45 μm or less, the outer diameter of the first common cladding be 60 μm or more and 85 μm or less, and the outer diameter of the second common cladding be 124 μm or more and 126 μm or less. In this case, it is possible to suppress the residual of the tensile stress on each core, and realize low transmission loss, high relative refractive index difference, and low fusion splice loss.

[0032] (7) As one embodiment of the present disclosure, it is preferable that the inter-core crosstalk in the two cores be -30 dB / 100 km or more and -18 dB / 100 km or less at a wavelength of 1550 nm. In this case, it is possible to realize long distance and large capacity transmission.

[0033] In the above, each of the modes listed in the [Explanation of Embodiments of the Present Disclosure] section can be applied to each of the remaining modes or all combinations of the remaining modes.

[0034] [Details of Embodiments of the Present Disclosure]

[0035] Hereinafter, the detailed structure of a specific example of an MCF (multi-core fiber) according to an embodiment of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to these examples, but is represented by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In addition, in the description of the drawings, the same reference numerals are attached to the same elements, and repeated description is omitted.

[0036] (Definition of Terms)

[0037] (a) In the present specification, except for the places specifically described, when the refractive index of a certain medium is set to n and the refractive index of pure silica glass is set to n0, the relative refractive index difference Δ of the medium is given by the following formula (1):

[0038] Δ = (n / n0) - 1... (1)

[0039] Definition.

[0040] (b) When the radial coordinate of an optical fiber is set to r, the following formula (2) is used:

[0041] Δ = Δ0 + (Δ1 - Δ0) x ((r - r0) / (r1 - r0)) α01 ... (2)

[0042] When approximating the relative refractive index difference Δ between the inner radius r0 and the outer radius r1, α01 is referred to as the index α of the region (hereinafter referred to as the "α value"). Δ0 and Δ1 are the relative refractive index differences at the two ends of the region, i.e., the radius r = r0 and r1, respectively, and α01 is determined using the least squares method.

[0043] (c) The "concentration" of a substance added to silica glass is expressed as mass concentration unless otherwise specified. That is, the concentration of the added substance in silica glass is expressed as the ratio of the weight of the added substance in a specific region to the weight of the silica glass in that specific region.

[0044] (d) In an MCF, "crosstalk between cores when propagating in the same direction" means the ratio of the power of light that is incident on a core located at one end of the MCF and emitted from one of the other cores located at the other end of the MCF to the power of light that is incident on the core located at one end of the MCF and emitted from the other end of the same core.

[0045] (e) In an MCF, "inter-core crosstalk during counter-propagation" refers to the ratio of the power of light incident on a core at one end of the MCF and emitted from one of the other cores at the same end, relative to the power of light incident on the core at one end of the MCF and emitted from the other end of the same core. It should be noted that in this specification, when simply referred to as "inter-core crosstalk," it refers to inter-core crosstalk during co-directional propagation.

[0046] (f) The spacing between two cores (hereinafter referred to as "inter-core spacing") is defined as the distance between the refractive index centroids of the cores. It should be noted that the refractive index centroid (X, Y) of a region S refers to the position of the centroid weighted by the refractive index n(x, y) within that region:

[0047] (Mathematical formula 1)

[0048]

[0049] (g) The "non-circular state" of the cross section of the cladding, etc. (defined on a plane perpendicular to the fiber axis corresponding to the central axis of the optical fiber) is expressed by the following formula (3):

[0050] 2×(long diameter - short diameter) / (long diameter + short diameter)…(3)

[0051] definition.

[0052] (h) The “transition region” is a region defined on the cross section of the optical fiber, and is a region having a width of 1 μm to 5 μm including the core-cladding boundary between each core and the cladding in each of the plurality of cores.

[0053] (i) "Connection region" is a region defined on a cross section of an optical fiber, constituting a part of each core and a cladding surrounding the same, and is a region in the cladding having a width of 0.5 μm or more and 2.5 μm or less in direct contact with each core.

[0054] (Embodiment)

[0055] Figure 1 An MCF1 according to an embodiment of the present disclosure is shown. The MCF1 includes a glass fiber 100 extending along a central axis (optical fiber axis) AX, a first cladding 30 surrounding an outer periphery of the glass fiber 100, and a second cladding 40 surrounding an outer periphery of the first cladding 30. The glass fiber 100 includes two cores 10a, 10b each extending along the central axis AX, and a common cladding 20 surrounding the two cores 10a, 10b. In addition, the common cladding 20 is composed of a first common cladding 20a (inner cladding) directly surrounding the two cores 10a, 10b, and a second common cladding 20b (outer cladding) surrounding the first common cladding 20a.

[0056] The two cores 10a, 10b and the common cladding 20 are each composed of silica glass, and can contain a halogen element such as a chlorine element (Cl), a fluorine element (F), a bromine element (Br), or the like. Each of the two cores 10a, 10b contains Cl at a concentration of 10,000 ppm or more, and thus has a relative refractive index difference of 0.10% or more with respect to pure silica glass. More preferably, each of the cores 10a, 10b contains Cl at a concentration of 15,000 ppm or more, and has a relative refractive index difference of 0.15% or more. Further preferably, each of the cores 10a, 10b contains Cl at a concentration of 20,000 ppm or more, and has a relative refractive index difference of 0.20% or more. On the other hand, the concentration of Cl in each of the cores 10a, 10b is 30,000 ppm or less, and thus the generation of bubbles in the manufacturing process of the optical fiber can be suppressed (as a result, high productivity of the optical fiber can be achieved). In addition, each of the cores 10a, 10b can contain one or more elements other than Cl, such as F and Br, and thus it is preferable to reduce the viscosity of each of the cores 10a, 10b from the viewpoint of manufacturability. The common cladding 20 has a refractive index lower than that of either of the cores 10a, 10b, and has a relative refractive index difference of -0.50% or more and -0.20% or less with respect to the refractive index of each of the cores 10a, 10b in a region having a width of 10 μm with the core-cladding boundary (the boundary between each of the cores 10a, 10b and the common cladding 20) as an inner periphery.

[0057] The common cladding 20 is made of pure silica glass or silica glass containing F. The F concentration is 0 ppm or more and 11,000 ppm or less, whereby the relative refractive index difference is -0.36% or more and 0% or less with respect to the refractive index of pure silica glass. More preferably, the F concentration is 0 ppm or more and 7,500 ppm or less, whereby the relative refractive index difference is -0.25% or more and 0% or less. According to the above structure, the F concentration in the cladding can be intentionally reduced as compared with the dual-core optical fiber of the above Non-Patent Literature 3, and the transmission loss of the optical fiber and the manufacturing cost can be reduced.

[0058] In addition, the cores 10a, 10b each have a diameter of 6 μm or more and 16 μm or less, whereby the cable cutoff wavelength of 1,530 nm or less in wavelength is obtained. More preferably, the cores 10a, 10b each have a cable cutoff wavelength of 1,480 nm or less in wavelength.

[0059] The outer diameter of the common cladding 20 is preferably a standard outer diameter of 124 μm or more and 126 μm or less. The non-circularity of the cross section of the common cladding 20 can be 0.1% or more and 2.0% or less, and the cross section of the common cladding 20 is not circular, whereby the measurement accuracy of the rotation angle can be improved when rotation centering is performed for fusion splicing. More preferably, the non-circularity of the cross section of the common cladding 20 is 0.2% or more and 1.0% or less.

[0060] The first coating 30 and the second coating 40 are made of an ultraviolet-curable resin of a urethane acrylate type. The first coating preferably has a thickness of 15 μm or more and 40 μm or less and has an elastic modulus of 0.05 MPa or more and 0.5 MPa or less. The second coating 40 preferably has a thickness of 15 μm or more and 40 μm or less and has an elastic modulus of 500 MPa or more and 2,000 MPa or less. Thereby, external force applied from the side of the optical fiber can be effectively shielded, and microbend loss due to glass deformation in the cores 10a, 10b and the common cladding 20 can be suppressed.

[0061] Figure 2 The cross-sectional structure, Cl concentration, F concentration, and refractive index profile of the glass fiber 100A (corresponding to the glass fiber 100 of the first embodiment of the present disclosure) included in the MCF according to the first embodiment of the present disclosure are shown. Note that, in the following, the glass fiber 100A is referred to as the glass fiber 100. Figure 1 Figure 2 ​In the figure, the Cl concentration is represented by [Cl], [Cl]0 represents the maximum Cl concentration of the cores 10a, 10b, and [Cl]1 represents the Cl concentration at the core-clad boundary. The F concentration is represented by [F], [F]1 represents the maximum F concentration in the first common clad layer 20a in the common clad layer 20, [F]2 represents the average F concentration in the second common clad layer 20b in the common clad layer 20, and [F]10 represents the F concentration at the core-clad boundary. In addition, the refractive index profile is defined by the relative refractive index difference of each portion on the line L1 passing through the center axis AX in the cross section of the glass fiber 100A, Δ0 is the maximum relative refractive index difference of the cores 10a, 10b, Δ1 is the minimum relative refractive index difference in the first common clad layer 20a, and Δ2 is the average relative refractive index difference in the second common clad layer 20b.

[0062] The distance to the outer edge of the clad layer can be maximized by arranging two cores on the symmetry axis to reduce transmission loss, and the accuracy of the rotation angle at the centering can be improved by arranging the cores off the symmetry axis. Note that, Figure 2 The cross-sectional structure of the glass fiber 100A shown in the figure is substantially the same as that of the glass fiber 100 shown in the figure, Figure 1 The cross-sectional structure of the glass fiber 100A shown in the figure is substantially the same as that of the glass fiber 100 shown in the figure,

[0063] In the glass fiber 100A, Cl is added to the cores 10a, 10b, and the Cl concentration [Cl] forms a substantially flat concentration distribution at the center of each of the cores 10a, 10b at a concentration [Cl]0, and continuously decreases to a concentration [Cl]1 at the core-clad boundary. As a result, the relative refractive index difference of each of the cores 10a, 10b with respect to pure silica is Δ0 at the center and Δ1 at the core-clad boundary. The upper limit of the concentration [Cl]0 is 10000 ppm, and Δ0 is 0.10%. It is preferable that the upper limit of the concentration [Cl]0 be 15000 ppm, and Δ0 be 0.15%. It is further preferable that the upper limit of the concentration [Cl]0 be 20000 ppm, and Δ0 be 0.20%. The upper limit of the concentration [Cl]1 is 2000 ppm, and Δ1 is 0.02%. It is preferable that the upper limit of the concentration [Cl]1 be 1000 ppm, and Δ1 be 0.01%. It is more preferable that the upper limit of the concentration [Cl]1 be 0 ppm, and Δ1 be 0%.

[0064] A core having a Cl concentration distribution as described above can be achieved by increasing the specific surface area of ​​a glass soot body manufactured by VAD (Vapor-phase Axial Deposition) or OVD (Outside Vapor Deposition) in a chlorine-containing gas atmosphere such as Cl2 or SiCl4, or by increasing the partial pressure of the chlorine-containing gas during sintering to the same level as or several times the atmospheric pressure. In particular, in a hollow soot body manufactured by the OVD method, it is preferred to form a Cl concentration distribution that decreases toward the periphery by making the chlorine-containing gas concentration in the hollow portion higher than that in the peripheral portion. Specifically, the α value when approximating the distribution of the relative refractive index difference due to the addition of Cl as the entire core using the above formula (2) is preferably 50 or less, and more preferably 20 or less. As a result, bubbles caused by the vaporization of Cl compounds can be suppressed near the core-cladding boundary, preventing a decrease in the yield of the base material process and the drawing process. On the other hand, if the change in the Cl concentration distribution at the core-cladding boundary is slowed, the core-cladding boundary may become blurred when the optical fiber is viewed from the side during fusion splicing, potentially reducing the accuracy of positional or angular alignment. Therefore, when approximating the refractive index distribution of the entire core using the above equation (2), the α value is preferably 3 or greater, and more preferably 5 or greater.

[0065] The core spacing is 45 μm, preferably 40 μm, more preferably 35 μm, and further preferably 30 μm. By narrowing the core spacing, each core can be concentrated at the cross-sectional center of the optical fiber. Thus, when the optical fiber is cut and fused, even if the edge of the cladding periphery of the end face becomes rounded (deformation caused by surface tension when the glass is softened by heating), the deformation of the core can be suppressed. In addition, since the stress is unstable near the cladding periphery, the flatness of the end face when the optical fiber is cut is often reduced, but according to this embodiment, the reduction in the flatness of the core can be effectively suppressed. Furthermore, the core position deviation caused by the adjustment error of the rotation angle can also be reduced. As a result, the fusion loss is suppressed to less than 0.10 dB, preferably less than 0.08 dB, more preferably less than 0.06 dB, and further preferably less than 0.04 dB.

[0066] The first common cladding 20a is doped with pure silica glass or F at a concentration of [F]1, resulting in a relative refractive index difference of Δ1 relative to pure silica. The second common cladding 20b is doped with F at a concentration of [F]2, resulting in a relative refractive index difference of Δ2 relative to pure silica. The F concentration [F] in the first common cladding 20a decreases continuously toward the core in the region with a width of 0.5 μm or more and 2.5 μm or less (the connection region of the common cladding 20) that is in direct contact with each core, to a concentration of [F]10, where the relative refractive index difference relative to pure silica decreases to Δ10 (although not shown, this is the relative refractive index difference at the core-cladding boundary). The upper limit of the concentration [F]1 is 11,000 ppm, resulting in a Δ1 of -0.36%. The preferred upper limit of the concentration [F]1 is 7,500 ppm, resulting in a Δ1 of -0.25%. The more preferred upper limit of the concentration [F]1 is 0, resulting in a Δ1 of 0. The upper limit of the concentration [F]10 is 7500 ppm, and the preferred upper limit is 5000 ppm. Furthermore, the lower limit of [F]1-[F]10 is 1500 ppm, resulting in a Δ1-Δ10 ratio of -0.05%. The preferred lower limit of [F]1-[F]10 is 3000 ppm, resulting in a Δ1-Δ10 ratio of -0.10%. Increasing Δ1-Δ10 can simultaneously suppress the increase in loss due to interface bubbles and Rayleigh scattering caused by the addition of fluorine, and suppress bending loss by strongly confining light. A more preferred upper limit of the concentration [F]10 is 0 ppm.

[0067] Furthermore, the connection region (the region in contact with each core, with a width of 0.5 μm or more and 2.5 μm or less) within the first common cladding 20a significantly affects optical properties and is therefore important. The average relative refractive index difference in this connection region is set to Δ1avg, with the lower limit of Δ0-Δ1avg preferably being 0.36%. A more preferred lower limit of Δ0-Δ1avg is 0.32%. Even more preferred is a lower limit of Δ0-Δ1avg of 0.28%. The outer diameters of the cores 10a and 10b are preferably 6 μm or more and 16 μm or less. Consequently, the cores 10a and 10b each have a cable cutoff wavelength of 1530 nm or less, maintaining compatibility with existing submarine cable systems. More preferably, the outer diameters of the cores 10a and 10b are 6 μm or more and 14 μm or less. Consequently, the cores 10a and 10b each have a cable cutoff wavelength of 1480 nm or less, enabling wavelength-multiplexed transmission over a wider wavelength band. The cable cutoff wavelength is preferably 1260 nm or greater, which reduces the inter-core crosstalk during co-directional propagation to -18 dB / 100 km or less (equivalent to -42 dB / 100 km or less during counter-directional propagation), making it suitable for long-distance transmission. More preferably, the cable cutoff wavelength is 1310 nm or greater, which reduces the inter-core crosstalk during co-directional propagation to -20 dB / 100 km or less (equivalent to -44 dB / 100 km or less during counter-directional propagation), making it even more suitable for long-distance transmission.

[0068] The concentration [F]2 is lower than the concentration [F]1 by 900 ppm or more, and Δ2 is higher than Δ1 by 0.03% or more. Preferably, the concentration [F]2 is lower than the concentration [F]1 by 1500 ppm or more, and Δ2 is higher than Δ1 by 0.05% or more. More preferably, the concentration [F]2 is lower than the concentration [F]1 by 3000 ppm or more, and Δ2 is higher than Δ1 by 0.10% or more. Thus, the viscosity of the second common cladding layer 20b is increased, and the increase in transmission loss and the decrease in core-cladding relative refractive index difference due to the tensile stress remaining in the cores 10a, 10b respectively caused by the tension at the time of drawing are able to be prevented. In order to suppress the increase in transmission loss, the tensile stress remaining in all the cores is preferably 10 MPa or less, and more preferably is set to 0 MPa or less, i.e., a compressive stress. The outer diameter of the first common cladding layer 20a is 85 μm or less, and is preferably 60 μm or less. Thus, the area of the second common cladding layer 20b is able to be sufficiently increased, and the stress caused by the tension at the time of drawing remaining on the cores 10a, 10b is able to be reliably suppressed. As such, by adding a high concentration of Cl to the cores to reduce the viscosity of the cores, and by managing the addition amount of F added in order to reduce the refractive index of the cladding layer, the increase in transmission loss due to the tensile stress remaining in the cores caused by the tension at the time of drawing is able to be suppressed, as also described in the above Patent Document 1, but unlike the above Patent Document 1, in the present disclosure, by making the cores a plurality of cores, the plurality of cores are able to share the drawing tension, and the remaining of the tensile stress is able to be more effectively suppressed. In addition, the addition amount of Cl in the cores needed in order to obtain the same transmission loss is able to be reduced, and thus the reduction in yield due to the generation of bubbles of Cl in the parent material is able to be reduced.

[0069] Preferably, the concentration [F]10 is 70% or less with respect to the concentration [F]1. More preferably, the concentration [F]10 is 50% or less with respect to the concentration [F]1. Thus, the generation of bubbles at the core-cladding interface due to the vaporization of the F compound is able to be suppressed, and the parent material and the optical fiber are able to be manufactured at a high yield.

[0070] Figure 3 The cross-sectional structure, the Cl concentration, the F concentration, and the refractive index profile of the glass fiber 100B (corresponding to the glass fiber 100 of the second embodiment of the present disclosure) included in the MCF according to the second embodiment of the present disclosure are shown. Figure 1 Figure 3 ​In the graph, the Cl concentration is represented by [Cl], [Cl]0a represents the maximum Cl concentration in the core 10a, [Cl]0b represents the maximum Cl concentration in the core 10b, and [Cl]1 represents the Cl concentration at the core-cladding boundary. The F concentration is represented by [F], [F]1 represents the maximum F concentration in the first common cladding 20a of the common cladding 20, [F]2 represents the average F concentration in the second common cladding 20b of the common cladding 20, and [F]10 represents the F concentration at the core-cladding boundary. Furthermore, the refractive index profile is defined by the relative refractive index differences of the respective portions on a line L2 passing through the central axis AX in a cross section of the glass fiber 100B, Δ0a represents the maximum relative refractive index difference in the core 10a, Δ0b represents the maximum relative refractive index difference in the core 10b, Δ1 represents the minimum relative refractive index difference in the first common cladding 20a, and Δ2 represents the average relative refractive index difference in the second common cladding 20b.

[0071] in addition, Figure 3 The cross-sectional structure of the glass fiber 100B shown in FIG. Figure 2 The cross-sectional structure of the glass fiber 100A shown in FIG is substantially the same, where P1 represents the core spacing, P2 represents the outer diameter of the first common cladding 20a, and P3 represents the outer diameter of the second common cladding 20b. However, in the glass fiber 100B, the Cl concentration of the core 10a is different from that of the core 10b. As a result, the relative refractive index difference of the core 10a is different from that of the core 10b. In this regard, the glass fiber 100B ( Figure 3 ) structure and glass fiber 100A ( Figure 2 ) has a different structure. Specifically, the Cl concentration of core 10a is [Cl]0a, and the relative refractive index difference is Δ0a. Furthermore, the Cl concentration of core 10b is [Cl]0b, and the relative refractive index difference is Δ0b. |[Cl]0a-[Cl]0b| (expressed in absolute value) is 2000 ppm or more, and |Δ0a-Δ0b| is 0.02% or more. Preferably, |[Cl]0a-[Cl]0b| is 5000 ppm or more, and |Δ0a-Δ0b| is 0.05% or more. As a result, a propagation constant difference is formed between the two cores 10a and 10b, and the inter-core crosstalk during propagation in the same direction is less than -21dB / 100km (equivalent to the inter-core crosstalk during propagation in opposite directions: less than -45dB / 100km). More preferably, the inter-core crosstalk during propagation in the same direction is less than -24dB / 100km (equivalent to the inter-core crosstalk during propagation in opposite directions: less than -48dB / 100km), which enables long-distance transmission with a high signal-to-noise ratio.

[0072] Description of Reference Numerals

[0073] 1: MCF (multi-core fiber); 10a, 10b: core; 20: common cladding; 20a: first common cladding (inner side cladding); 20b: second common cladding (outer side cladding); 30: first coating (resin coating); 40: second coating (resin coating); 100, 100A, 100B: glass fiber.

Claims

1. A multi-core optical fiber having: multiple cores composed of silica glass; a common cladding composed of silica glass and surrounding the plurality of cores, the common cladding having a refractive index lower than that of each of the plurality of cores; and a resin cladding surrounding the common cladding, Each of the plurality of cores contains 10,000 ppm or more of chlorine in at least a portion thereof, At the center of each of the plurality of cores, the chlorine forms a flat concentration distribution. The concentration of chlorine in the core-cladding boundary between each of the plurality of cores and the common cladding is 2000 ppm or less, In a transition region having a width of 1 μm to 5 μm inclusive and including the core-cladding boundary in each of the plurality of cores, the concentration of chlorine continuously decreases in a direction away from a center of gravity of a refractive index of each of the plurality of cores.

2. The multi-core optical fiber according to claim 1, wherein: said common cladding comprises fluorine, In connection regions of the common cladding layer having a width of 0.5 μm to 2.5 μm, each directly contacting the plurality of cores, the fluorine concentration continuously decreases toward a direction approaching a center of gravity of a refractive index of each of the plurality of cores.

3. The multi-core optical fiber according to claim 2, wherein: The concentration of fluorine in the core-cladding boundary between each of the plurality of cores and the common cladding is 11000 ppm or less.

4. The multi-core optical fiber according to claim 2, wherein: The public cladding comprises: a first common cladding directly surrounding each of the plurality of cores; and a second common envelope surrounding said first common envelope, An average concentration of the fluorine in the first common cladding is greater than an average concentration of the fluorine in the second common cladding by 900 ppm or more.

5. The multi-core optical fiber according to claim 4, wherein: comprising two cores as the plurality of cores, The inter-core distance between the two cores is 30 μm or more and 45 μm or less, The outer diameter of the first common cladding is greater than or equal to 60 μm and less than or equal to 85 μm. The outer diameter of the second common cladding is greater than or equal to 124 μm and less than or equal to 126 μm.

6. The multi-core optical fiber according to claim 5, wherein: The inter-core crosstalk during the same-direction propagation in the two cores is not less than -30 dB / 100 km and not more than -18 dB / 100 km at a wavelength of 1550 nm.

Images